One difficulty in feeding granular or fine coal with moderate amounts of moisture into a high pressure gasifier includes devising a reliable way to increase the pressure of the coal stream from atmospheric pressure to the operating pressure while accurately metering the coal feed rate. Essentially, three methods have been practiced: slurry coal feed systems, dry powder feed systems with a lock vessel and rotating metering devices, and coal pumps.
For the slurry coal feeder, the slurry includes 32-40 wt % water added to the powder coal to make the coal slurry viscosity low enough for pumping into a high pressure gasifier. A fundamental issue with this approach is that the cost of drying the low rank coals to a nearly moisture-free condition is very high because the drying process can be quite lengthy. For example, U.S. Pat. No. 6,162,265 discusses drying and preparing a slurry with low rank coals. The process is quite complicated and increases the capital, operating and maintenance costs to a level where the gasification facility becomes economically unattractive. It is relatively easy to remove the surface moisture from the coal; however, the drying facility required to remove nearly all the moisture including inherent moisture from coal is quite large. After removing nearly all the moisture from the coal, water is added again to make the slurry. In addition to the high cost of drying the coal, the thermal efficiency of the gasification process will be low because the water added to slurry the nearly dry coal needs to be evaporated in the gasifier. Thus, in the combined process of coal drying and slurry feeding, the moisture in and with the coal undergoes evaporation twice. The energy required to evaporate the water in the gasifier increases the oxygen consumption as well as the associated capital and operating costs. If the coal is not dried before slurrying, then the amount of water that needs to be evaporated in the gasifier nearly doubles as the low rank coals contain significantly high levels of moisture, which can be in the range 30 to 45 percent by weight of the coal. The double duty moisture evaporation in the gasifier significantly lowers process efficiency and useful syngas yield and increases operational costs with high oxygen consumption and higher cooling duties to condense moisture from the syngas. Therefore, using the slurry method to feed low rank coals to a high pressure gasifier is economically unfeasible. This may be the fundamental reason why no known commercial practice exists.
Using a lock vessel, as has been operated in the prior art, to feed the granular coal and metering the coal with a rotating device such as an auger or a screw feeder encounters problems too. The lock vessel is a means to increase the pressure as the coal moves from the atmospheric pressure in the storage bin to the operating pressure in the feed vessels. The lock vessel method to increase the pressure is based on swinging the pressure between the atmospheric and operating pressures cyclically. The lock vessel has two valves: one connecting the atmospheric pressure storage bin from which coal is fed to the lock vessel and the other connecting to a coal feed vessel which receives coal from the lock vessel and is maintained at the operating pressure of the gasifier. In cyclic operation, when the lock vessel is ready to receive coal from the atmospheric vessel, the inlet valve opens. Once the coal reaches a predetermined level, the inlet valve closes. Then the lock vessel is pressurized using nitrogen or any other relatively inert gas such as CO2. When the lock vessel pressure equals to the feed vessel pressure, the outlet valve opens. The coal in the lock vessel is supposed to flow into the feed vessel under gravity. And most times, it does. However, when the coal moisture increases to above 5%, finely ground coal particles tend to pack in the lock vessel when the lock vessel is pressurized. The packed coal in the lock vessel will not dislodge from the lock vessel into the pressurized feed vessel. The feed vessel will gradually empty out, eventually interrupting the coal feed to the gasifier. The loss of coal feed affects the gasifier operation because it has to be shut down and restarted, thereby increasing operational costs and causing a loss in production.
The metering devices are generally rotating devices that require an appropriate seal to prevent the coal fines from leaking to the atmosphere. It is difficult to develop a long lasting reliable seal mechanically for the high pressure differences that exist between the feeder and the atmosphere because of the presence of coal dust and constant rotation of the device. Further, the accuracy of the metering becomes difficult because of the changes in coal bulk density. Erosion of the rotating device and housing by the coal particles is a significant issue for certain types of coal feeders.
Using a coal pump to directly feed coal to a high pressure gasifier has been in development for many years; but, the device has not been developed sufficiently enough for commercial applications. The fundamental issue is the wide variance in the properties of the coal powder or granular material. As the pressure seals are designed for certain narrow range of coal particles, it is difficult to seal at high pressures all the time for naturally varying feed characteristics of ground coal. The ground coal properties vary due to variations in raw coal, variations in crushing and milling operations and segregation of coal particles during storage and conveying. Any interruption of feed or blowback of hot ignitable process gases and gasifier bed materials can be highly unsafe for the coal pump system.